A document scanner is a device which converts a visible image such as a photograph, transparency or printed paper into an electronic form suitable for copying, storing or processing by a computer. Reflective document scanners typically have a controlled source of light which is reflected off the surface of a document onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency scanners pass light through a transparent image such as a photographic positive slide image and then onto an array of photosensitive devices.
A transparent image may be scanned in a reflective scanner by placing a white surface behind the transparency. In such an arrangement, light passes through the transparency to the white background, reflects off the white background and passes through the transparency a second time before impinging onto the photosensitive devices. Therefore, light is filtered twice by the transparent image. In general, this double filtering creams an unacceptable distortion of gray scale, color, contrast and other image characteristics of interest.
Reflective document scanners may be adapted to scan transparent images by providing a separate light source to back light the image so that the light passes through the transparency only once. In some systems, separate optics may be required to properly project the image onto the photosensitive devices. Motors may also be required to move the external light source relative to the image. There is a need for an adapter for scanning transparent images in a reflective document scanner in which the adapter is completely passive, with no separate light source and no moving parts, and in which light passes through the image only once.